1. Field of the Invention
This invention relates to a switching power source device capable of inhibiting harmonic distortion of a, ac power source.
2. Description of the Prior Art
Heretofore, as countermeasures for diminishing the harmonic distortion of a commercial ac input power source, an ac reactor inserting system, an active smoothing filter system and a transformer system, have been considered, as stated in "DENKI KYODO KENKYU", vol. 46, No. 2.
By the above-mentioned ac reactor inserting system is meant a system in which an ac reactor is inserted into an ac input side of a full-wave rectifier-capacitor smoothing circuit, a typical component used in general-purpose household electric equipment, for limiting the charging current to a capacitor by an impedance component of the ac reactor for diminishing high harmonic components by extending an angle of conduction.
By the above-mentioned active smoothing filter system is meant a system in which a booster type chopper converter, a non-insulator type switching regulator having an output voltage higher than an input voltage, is used in place of the above-mentioned full-wave rectifier-capacitor smoothing circuit. In operation, pulse signals, full-wave rectified by a bridge rectifier, are switched over an entire period at a frequency higher than several tens of kilohertzes. The result is that the input current waveform is averaged out for each of the periods of the switching current, such that, if a large capacitor be present in a load, such capacitor proves to be a resistance load, so that the input switching current is caused to flow sinusoidally to allow to diminish the high harmonics.
Finally, by the transformer type system is meant a system in which reduction of high harmonics may be achieved by the choking effects proper to the transformer and the enlarged current conduction angle due to the lower secondary side voltage.
However, the above-mentioned three conventional systems suffer from respective drawbacks.
With the ac reactor inserting system, the reactor device is bulky and heavy in weight, while being expensive. An ac reactor is expensive when compared to other components. Since the dc voltage derived from smoothing of the corresponding ac voltage is lowered so that redesigning in a downstream side switch regulator becomes necessary to lower the efficiency. Besides, electronic appliances may be affected by the stray magnetic flux from the reactor device.
With the above-mentioned active smoothing filter system, the noise level is increased due to electro-magnetic interference (EMI) derived from switching semiconductors. Circuitry becomes complicated due to switching controlling means for providing for an input voltage proportionate to an input electric current, starting circuit and to software functions, besides the function as a switching power source, thereby increasing the number of component parts and production costs. Insulation must be made by a downstream side switching regulator because the system is a non-insulated system.
Although the transformer system is limited to a small-capacity electronic equipment of less than 30 W, the equipment is increased in size in order to be commercialized.
If the defects of the three systems is inspected, it becomes apparent that, in view of avoiding the increase in size of the device, the ac reactor inserting system or the transformer system is not satisfactory under the current state of the art in respect of practical application. However, the active smoothing filter may lend itself to practical application, if integration of the active smoothing filter and the switching filter is achieved satisfactorily.
With the active filter circuit employing the above-mentioned active smoothing filter circuit, the circuitry is such that, as disclosed in "DENSHI GIJUTSU" extra issue of March 1990, a non-insulated booster chopper circuit is of a pulse width modulation (PWM) system with a fixed switching frequency or of a ringing choke converter (RCC) system with a variable switching frequency. The active filter circuit has a drawback that, since the switching semiconductor undergoes a repeated on-off switching operations with trapezoidal or triangular waves so that the electro-magnetic wave interference level derived from the semiconductors is increased. The active filter circuit also has drawbacks that, if the system is of an insulated type, it becomes a flyback converter to increase power losses and noise levels, while the circuitry becomes complex due to provision of a starting circuit, software functions and means for providing for the input voltage proportionate to the input current, besides the function of the switching power source, thus leading to increased number of component parts and increased costs.
With the switching power source system for suppressing the ac ripple voltage to less than 50 mV against load fluctuations and changes in the ac input voltages for maintaining a constant dc output voltage, a large capacity electrolytic capacitor is employed as an ac input rectifying smoothing capacitor. If, however, the load power is 150 W and the capacitor for ac input rectification has a capacitance of 820 .mu.F, the ac line current containing a large quantity of high harmonics charging the capacitor will flow as indicated in FIG. 2 to produce waveform distortions of the commercial sinusoidal ac voltage, such that the power factor is as low as 0.5 to 0.7.